A cruise control system in a vehicle provides automatic speed control to enable the vehicle to maintain constant speed under variable driving conditions without driver's intervention. A conventional cruise control system in an electric vehicle controls an electric motor of the vehicle to request a torque value required to achieve a desired speed.
For example, U.S. Pat. No. 5,615,933 describes an electric vehicle having an electric motor propulsion system, a brake system and a control unit, which includes a motor control controlling the propulsion system, and a brake control for controlling the brake system. The motor propulsion system includes a three-phase AC electric motor that provides traction force to vehicle drive wheels. The motor control provides control commands to the electric motor. In particular, the motor control derives a torque command that defines the torque to be applied to the wheels by the motor. The motor control comprises an electronic cruise control system that receives a vehicle speed signal from a vehicle speed sensor. In response to the vehicle speed signal, the motor control issues a command to set the motor either into an accelerating or a braking mode to achieve a desired speed. In the accelerating mode, an accelerating torque command is issued by the motor control to request the motor to increase the torque applied to the wheels so as to reach the desired speed.
Under typical driving conditions, torque values required to achieve a desired speed are subject to wide variability with little, if any, long term predictability. Moreover, driving conditions, such as steep uphill grade or heavy vehicle load or the like, may impose limitations on available speed and acceleration.
Higher acceleration or greater speed may be required than the system can accommodate at maximum torque restricted by available motor performance. In particular, the voltage signal that the motor control needs to produce in order to request the torque required to achieve the desired speed may be greater than the supply voltage. Therefore, the motor would not be able to develop the required torque.
Accordingly, there is a need for a cruise control system that would be able to control the electric motor of a vehicle in order to produce high torque required to maintain the vehicle at a desired speed.
In addition, in a vehicle drive environment, wherein power availability is limited to an on-board supply, it is highly desirable to attain a high torque output capability at minimum power consumption. Motor structural arrangements described in the copending applications contribute to these objectives. As described in those applications, electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects occurring from interaction with other electromagnet members.
The above-identified copending application Ser. No. 10/173,610 describes a control system for a multiphase motor that compensates for variations in individual phase circuit elements. A high degree of precision controllability is obtained with each phase control loop closely matched with its corresponding winding and structure. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with parameters associated with the respective stator phase components. The phase windings are energized with current of sinusoidal waveform for high efficiency operation. The control system varies the output current to respond to, and accurately track, the torque command input.
The sinusoidal current waveform profile obtained with this commutation strategy can extend battery life through efficient operation. However, in vehicle driving operation there may be a need for torque capability in excess of that available from the most efficient control scheme. Typically, the power supply is rated for a maximum current discharge rate, for example, 10.0 amps. If the cruise control system requests a torque command that correlates to this maximum current draw, then the motor torque output for a sinusoidal current waveform profile is limited, for example, to approximately 54.0 Nm in a motor with a configuration such as described above.
The need thus exists for an electric vehicle cruise control system that is capable of adaptively controlling the electric motor to enable its operation with high efficiency yet can deliver increased torque output when required to maintain a desired speed.